1. Field of the Invention
The present invention relates to a brushless motor and a control circuit thereof to be mounted on various office automation instruments such as plane paper copiers, laser printers, and optical magnetic disk drives.
2. Related Art
A small DC motor to be mounted on an office automation instrument such as a plane paper copier and a printer is provided with a stator fixed to a bracket and a rotor oppositely placed and coaxially rotatably supported by the stator. A rotor magnet, with a predetermined number of magnetic poles, is fixed to a yoke of the rotor. When a predetermined power is supplied to the stator, the rotor rotates by electromagnetic action.
In case of a brushless motor which generates a comparatively large torque, segment-shaped ferrite magnets are generally used as the rotor magnet. More specifically, a predetermined number of ferrite magnets are annularly placed and fixed along with the yoke of the rotor, to thereby obtain the predetermined number of magnetic poles.
However, the brushless motor constructed as described above poses the following problems on forming the rotor magnet.
Segment-shaped magnets must be stuck together annularly along the yoke of the rotor. In order to arrange the magnets uniformly while keeping a constant interval circumferentially, an arrangement of the magnets must be performed with high precision by using a positioning jig, resulting in a troublesome work.
When the segment-shaped magnets are arranged along the yoke of the rotor without providing a constant interval, gaps are caused between the adjacent magnets. The existence of the gaps will contribute to increase the reluctance torque. For this reason, cogging is increased and the rotational fluctuation may be caused.
Further, in order to provide an index magnet for rotation detection and a multi-pole magnet for frequency generation (FG) integrally formed with the rotor magnet, a further work is required for fixing and positioning the magnets.
A brushless motor having the above-described structure and sensors for detecting the rotational position of the rotor has been known. In most cases, the brushless motor is controlled by an electronic circuit constituted in a semiconductor chip. In this case, the timing of generating the magnetic field at the stator side is controlled by detecting the position of the rotor by the sensors. Hall elements have been conventionally used as the sensors. FIG. 15 shows an example of a rotation control circuit adapted for a three-phase spindle motor. In FIG. 15, HE1 through HE3 denote Hall elements. Hall amplifiers HA1 through HA3 are connected to the Hall elements HE1 through HE3, respectively.
Each output of the Hall amplifiers HA1 through HA3 is connected to a control section 401 which controls exciting currents supplied to the three-phase stator coils u, v, w in accordance with the output signals of the Hall elements HE1 through HE3. The outputs of the control section 401 are connected to a driver section 402. Upon receiving the output signals from the control section 401, the driver section 402 supplies exciting currents to the stator coils u, v, w.
An output of a current detecting circuit (amplifier) 403 is also supplied to the control section 401. A negative input terminal of the current detecting circuit 403 is connected to one terminal of a resistor R1 through which an exciting current supplied from the driver section 402 to one of the stator coils u, v, w flows. A positive input terminal of the current detecting circuit 403 is connected to a node of series-connected resistors R2 and R3 provided between a power supply voltage Vcc and a ground voltage.
The current detecting circuit 403 receives a conversion voltage V of the exciting current converted by the resistor R1, and a reference voltage Vs obtained by dividing the power supply voltage Vcc by means of the resistors R2 and R3. When the motor overruns or the number of revolutions becomes extremely high by some reason, the exciting currents supplied to the coils u, v, w increase. The current detecting circuit 403 detects this increase and supplies a detection signal to the control section 401. Upon receiving the detection signal, the control section 401 ceases the supplement of the exciting currents to the coils u, v, w, to thereby stop the motor.
However, such a conventional rotation control circuit, particularly the current detecting circuit 403 has the following problem.
According to the current detecting circuit 403, the reference voltage Vs is obtained by dividing the power supply voltage Vcc by the resistors R2 and R3. The resistance values of the resistors R2 and R3 vary within a range of approximately 10%. For this reason, the reference voltage Vs changes in accordance with the variation of the resistance values and also highly depends on the fluctuation of the power supply voltage Vcc.
When such a brushless motor which is in a constant revolution is to be ceased within a predetermined period of time, a brake control (braking operation) is performed. According to a brake circuit of a conventional brushless motor a brake operation is performed at full torque. A rotational frequency is detected by one Hall sensor H1 of three Hall sensors H1, H2, H3 detecting a rotation of the rotor, as shown in FIG. 18. Upon detecting that the rotational frequency becomes 1/n of the rated frequency, a deceleration detection circuit 450 switches the brake circuit to further perform the brake operation with a small torque during a time interval T set by a delay circuit 451, to thereby smoothly stop the rotor.
According to the above structure, after the number of revolutions (1/n of the rated frequency) is detected and the delay time T has elapsed, the brake currents are cut off. However, the detected number of revolutions will change depending on the rated number N.sub.B of revolutions.
Assume now that as shown in FIG. 19 the delay time T is set to be tailored to the rated number of revolutions, and further brake operation is performed for the time T after the number of revolutions reaches N.sub.B /n by performing the brake operation by the full torque, to thereby completely cease the rotor. However, when the number of revolutions is N.sub.A larger than N.sub.B, even if the brake operation is performed by the full torque until the number of revolutions reaches N.sub.A /n and further brake operation is performed for the time T, the brake operation is insufficient and thus the rotor is not completely stopped. In contrast, when the rated number of revolutions is N.sub.C smaller than N.sub.B, the brake operation is performed at full torque until the number of revolutions reaches N.sub.C, and further brake operation is performed for the time T, the brake operation is excessive and thus the rotor rotates in reverse direction.
As described above, according to a conventional brake operation circuit, the brake operation may not be performed stably depending on the value of the rated number of revolutions.